Thorium is a chemical element with symbol Th and atomic number 90. A reactive actinide metal, its chemistry is dominated by the +4 oxidation state; it is silvery and tarnishes black in air, forming the dioxide. It is malleable and moderately hard, with a high melting point. Its most stable isotope, 232Th, is weakly radioactive with a half-life of 14.05 billion years, roughly the age of the universe: it decays very slowly to stable lead. Thorium and uranium are the only two significantly radioactive primordial elements. Thorium is chiefly refined from monazite sands as a byproduct of extracting rare earth metals. It was discovered in 1829 by the Norwegian mineralogist Morten Thrane Esmark and identified by the Swedish chemist Jöns Jacob Berzelius, who named it after Thor, the Norse god of thunder. Applications were found for the element in the 19th century, but after its radioactivity was discovered in the early 20th century, its use significantly declined. Thorium has been suggested as a replacement for uranium in nuclear reactors.

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